Washington ? Flanked by the leaders of Afghanistan and Pakistan, President Barack Obama expressed deep U.S. regret Wednesday for civilian casualties in a deadly incident this week in western Afghanistan, promising “every effort” to avoid recurrences in the war against a rising Taliban insurgency.

Obama had a more upbeat and determined tone as he lauded “unprecedented cooperation” between the two neighbors in fighting Taliban and other extremist threats. But he cautioned that success will not come quickly.

“Along the border, where insurgents often move freely, we must work together with a renewed sense of partnership to share intelligence and to coordinate our efforts to isolate, target, and take out our common enemy,” Obama said after a day of meetings with Afghan President Hamid Karzai and Pakistani President Asif Ali Zardari.

Obama met separately with Karzai and Zardari, followed by a three-way session that Secretary of State Hillary Rodham Clinton portrayed as an embodiment of a central tenet of the administration’s new Afghan war strategy — that Pakistan and Afghanistan are linked problems.

The latest report of Afghan civilian casualties came at an especially awkward time for the administration, which is stepping up its military campaign inside Afghanistan while also seeking to emphasize the importance of nonmilitary efforts to stabilize the country. The U.S. has pledged, for example, a major increase in civilian expertise in farming and other specialties, along with an increase of 21,000 U.S. troops.

Obama’s strategy, unveiled in March, already is threatened by setbacks to his goal of strengthening a shaky Pakistani government, eliminating al-Qaida and Taliban sanctuaries on the Pakistan side of the Afghan border and fighting Afghan government corruption. Claims of U.S. culpability for civilian deaths in Afghanistan are an added burden.

Gen. Jim Jones, Obama’s national security adviser, told reporters the president began his meeting with Karzai by addressing reports that dozens of civilians had been killed by American bombs on Sunday.

Jones said Obama commented “with great sympathy” and expressed regret for the loss of innocent life. Earlier, before her meetings with Karzai and Zardari at the State Department, Clinton said the U.S. “deeply, deeply” regrets the losses.

Both Obama and Clinton stopped short of accepting U.S. blame for the deaths.